Technical Field
The present disclosure relates generally to shape memory materials, and more particularly, to embodiments of a multi-shape product that can affix in a plurality of temporary shapes.
Description of Related Art
Shape memory materials include shape memory polymers (“SMPs”). Manipulation of these polymers can “fix” a temporary shape and later recover to an original, “memorized” permanent shape upon heating (See references 1-4 below). A typical shape memory cycle (“SMC”) involves first deforming the SMP in its rubbery state at an elevated temperature. This deformation is elastic in nature and mainly results in a decrease in conformational entropy of the constituent polymer network chains. Vitrification or crystallization, which is triggered by cooling the deformed material, kinetically traps the SMP in its low-entropy state due to a significant reduction of chain mobility. Macroscopically the material retains its temporary shape even after releasing the external stress. Reheating the material later initiates shape recovery under stress-free or loaded conditions. Heating allows relaxation of polymer chain segments (with regained mobility) to their original, entropically-favored conformational state.
The field of SMPs has grown rapidly. Much of this growth is due to the intrinsic versatility of SMPs which permits implementation of SMPs in many applications ranging from actuators to sensors to medical devices. A number of materials exhibit unprecedented properties that greatly extend the scope of traditional SMPs. With regard to triggering mechanisms (e.g., the external stimulus), in addition to direct heating, SMPs are also responsive to light, electricity, moisture, solvents, and magnetic fields (See, respectively, references 5-9 below).
Some SMPs have unique recovery characteristics. For example, “two-way” shape memory SMPs exhibit reversible actuation capabilities. These capabilities are reported for liquid crystalline elastomers (LCEs) and semi-crystalline networks (See references 10-12 below). Examples of “triple-shape” SMPs, discussed more below, exhibit two separate transitions with three different shapes (See references 10 and 13-16 below). However, unlike conventional “dual-shape” SMPs, which only recover from a temporary shape to a permanent shape, triple-shape SMPs can fix at least two temporary shapes and recover sequentially from one of the temporary shapes to the other and, eventually, to the permanent shape. Recovery occurs in response to applied stimuli, e.g., heating.
SMPs with triple-shape properties often feature two separate shape-fixing mechanisms. The mechanisms are distinguished, in one example, by separate thermal transitions in a temperature range associated with the application in which the material is implemented. The separate thermal transitions lead to a cascade of three elastic modulus plateaus, wherein each plateau is of decreasing magnitude in response to increasing temperature.
Various approaches are known to implement triple-shape properties and features in shape memory materials. These approaches occur at both the molecular and macroscopic level. At the molecular level, Bellin et al. disclose two copolymer networks (See references 14 and 15 below). These networks encompass (1) poly(ε-caprolactone) (PCL) segments with grafted, short poly(ethylene glycol) (PEG) side chains, and (2) main-chain poly(cyclohexyl methacrylate) (PCHMA) cross-linked with di-functional PCL macromers. The resulting shape memory material shows well-separated thermal transitions from (1) PCL Tm (50-60° C.) and PEG Tm (17-39° C.) and (2) PCL Tm (50-60° C.) and PCHMA Tg (˜140° C.) and can separately fix two temporary shapes in a programmed thermo-mechanical cycle.
Also at the molecular level, Mather et al. describe a homopolymer (rather than copolymer) such as a poly(2-tert-butyl-1,4,-bis[4-(4-pentenyloxy)benzoyl]hydroquinone) (P5tB) liquid crystalline network (“LCN”) (See reference 10 below). The LCN can fix two temporary shapes through first isotropic-nematic transition (˜150° C.) and then glass transition (˜80° C.). Heating the fixed sample leads to complete and sequential recovery of the temporary shapes in reverse order.
Macroscopically, Xie et al. report on triple-shape behavior in polymer bi-layers composed of epoxy thermosets with two different transition temperatures (e.g., glass transition temperature Tg) of 38° C. and 75° C. (See references 16 below). The triple-shape properties of these bi-layers are characterized under bending deformations and are shown to be easily tuned by varying the thickness ratio of the two layers. However, the intrinsic asymmetry of this approach may render the resulting shape memory material (and thus its shape memory properties) sensitive to the direction of the load.
Nonetheless, although the materials above exhibit triple-shape properties, each has limitations that may preclude or limit the applications for which and in which the materials can be applied.